Replace the return type of make_iterator from "iterator" to "typing.Iterator"

[AWhetter]

Issue description

Currently when declaring a method that returns the result of py::make_iterator(), the type signature in the docstring uses the return type iterator, which is not a valid Python type. In the example below we see the docstring:

__iter__(self: numbers.Numbers) -> iterator\n

I was hoping that the type could instead be switched to either typing.Iterator or Iterator.

__iter__(self: numbers.Numbers) -> typing.Iterator\n

The use case where this is coming up:
I'm generating type stubs of a pybind module using stubgen. When running mypy with the generated stubs I see the following error:

error: name 'iterator' is not defined

Reproducible example code

#include <pybind11/pybind11.h>

namespace py = pybind11;

static const std::vector<string> numbers = {"1", "2", "3"};

class Numbers {
    std::vector<int>::iterator begin() { return numbers.begin(); }
    std::vector<int>::iterator end() { return numbers.end(); }
}

PYBIND11_MODULE(numbers, m) {
    py::class<Numbers>(m, "Numbers")
        .def("__iter__",
            [](const Numbers& self) {
                return py::make_iterator(self.begin(), self.end());
            },
            py::keep_alive<0, 1>()
        )
    ;
}

>>> import numbers
>>> numbers.Numbers.__iter__.__doc__
'__iter__(self: numbers.Numbers) -> iterator\n'

[AWhetter]

I'm going to include this under the same issue because it seems very similar; the py::none type declares it's type in a docstring as none, but I would expected it to be None so that it produces a valid Python type.

[AWhetter]

I've since discovered that the Iterator problem has already been fixed in 57070fb
However "none" is still an issue.